Images from local newspaper Apple Daily showed the tap with a sign that said "Starbucks only" a few feet away from a urinal in the dingy washroom, which the paper said was in the building's carpark.

"Totally disappointed! The initial decision by Starbucks to use water from toilet is a clear sign of your company's vision and the level of (dis)respect your company has for the health and mind of your customers." Kevin L wrote on the Starbucks Hong Kong Facebook wall.

"I'm now really worried when I purchase coffee from Starbucks. Who knows which other stores are using the similar practice! Scary!!" Kevin L wrote.

"I stick to my McCafe and the water is not from a toilet!" Cristiane Guimaraes commented on her Facebook wall.

The water was collected less than five times a day by staff from a tap in a toilet located near the store, Starbucks told AFP, adding that it was dedicated for collecting drinking water.

"There is no direct water supply to that particular store, that's why we need to obtain the drinking water from the nearest source in the building," Starbucks spokeswoman Wendy Pang said.

The water from the toilet tap would go through a filtration system in the store ensuring it passed local and World Health Organisation standards, Pang said, adding the store is now using distilled water.

"The issue that is more worrying is...the potential risk of transferring pathogens from the restroom environment into the Starbucks food preparation area," Hong Kong University School of Public Health associate professor Benjamin Cowling said.

"I wouldn't go to the restaurant in the first place if I knew they were having potentially risky hygiene practices," Cowling said.